Glass and similar transparent substrates can be coated with transparent films to alter the optical properties of the glass. High visible transmittance, low emissivity coatings are characterized by their ability to transmit visible light while minimizing the transmittance of other wavelengths of light, particularly light in the infrared spectrum. This characteristic is useful for minimizing radiative heat transfer without impairing visibility, and coatings of this type find utility in architectural glass or automobile windows. It is often desired to maintain reflectance relatively consistent throughout the visible spectrum so that the coating has a “neutral” color; that is, colorless.
Generally speaking, coatings on glass that are provided for high transmittance and low emissivity comprise a stack of films having one or more thin metallic films with high infrared reflectance and low transmissivity that are disposed between antireflective dielectric layers that commonly are metal oxide films. The metal oxide layers serve to reduce visible reflection of the film stack to enhance transmittance, and are characterized by relatively high indices of refraction, commonly on the order of 1.9 or more.
Thin, transparent metal films of silver, copper and the like are susceptible to corrosion (e.g., staining) when they are brought into contact, under moist or wet conditions, with various staining agents such as atmosphere-carried chlorides, sulfides, sulfur dioxide and the like. Films of this type commonly are employed on inner surfaces of multi-pane glass units so that the films are maintained in a dry condition by desiccants or the like that remove moisture from the interpane spaces. Staining can occur when coated panes of glass are stored for later fabrication into insulating glass units.
Film stacks frequently are isolated from contact with the environment, and a film stack of the type described may be positioned on one of the inner surfaces of a multipane insulating glass unit. However, when glass panes bearing coating stacks are transported or assembled into multipane units, they often are subjected to relatively harsh conditions which may cause physical marring of the film stacks.
Film stacks commonly are provided on glass sheets on a commercial production basis through the use of magnetron sputtering techniques such as those described in Chapin, U.S. Pat. No. 4,166,018.
Gillery, et al. U.S. Pat. No. 4,786,563 suggests the use of a thin overcoat of titanium dioxide as a protective layer. Titanium oxide overcoats may be particularly prone to scratching or abrasion during shipping and washing operations, however, rendering the glass panes commercially unsuitable for use. O'Shaughnessy, et al. U.S. Pat. No. 5,296,302 corrects the problem by providing a protective overcoat of an oxide such as zinc oxide, the latter being relatively very thin in comparison to other films in the stack, and protective overcoats of this type having thicknesses in the range of 10–40 Å are disclosed.
High transmittance, low emissivity film stacks of the type described in U.S. Pat. No. 5,296,302, despite their excellent resistance to scratching, nonetheless have experienced problems in connection with the tarnishing or other discoloration of the reflective metal layers, which commonly are silver. Moreover, since the sputter deposition of certain films such as titanium oxide proceeds more slowly than zinc oxide, for example, it would be desirable to avoid the presence of titanium oxide films of thicknesses greater than, for example, 30 Å.
It would be desirable to provide such film stacks with protection not only against physical damage (e.g., scratching) but also against tarnishing or discoloration of the metal reflective layers employed in such film stacks.